In a radio frequency (RF) system, the efficiency of a power amplifier can be an important aspect of the function of the system. To assist in improving the efficiency of a power amplifier in an RF system, the baseband signal can be clipped to reduce the peak-to-average ratio (PAR) before modulation to the RF band. The process of clipping the baseband is generally referred to as crest factor reduction (CFR). A simple clipping algorithm can be sufficient for some applications, but it does generate a noise floor across the frequency.
In some applications, such as wideband code division multiple access (WCDMA), a higher adjacent channel leakage ratio (ALCR) compared to the in-band signal noise floor is employed. For example, a WCDMA system usually requires 65 dB ALCR and −25 dB error vector magnitude (EVM). One way to achieve this is through the use of a cancellation pulse (CP) algorithm that can accurately achieve the desired PAR and ALCR requirements.
For CP-based systems, though, “superpeak” situations can occur. In a multi-carrier narrowband signal, a closely spaced group of signals can appear as a “superpeak.” Looking to FIG. 1A as an example, a base CP with a bandwidth of 1.25 MHz and a sampling frequency of 61.44 MHz is shown, where CP0 denotes the CP with a center frequency at 0 and CP6 denotes the CP with a center frequency at 7.5 MHz. Thus, as can be seen in FIG. 1B, a “superpeak” that receives contributions from CP0 and CP6 can be seen.
These “superpeaks” can further result in “peak re-growth,” where peaks re-grow even after a cancellation algorithm is applied. The new peaks resulting from the re-growth can be larger in amplitude than the original peak. A peak re-growth condition is illustrated in FIG. 1C. Assuming that there are three carriers at −7.5 MHz, 0 Hz, and 7.5 MHz and that the peak is only contributed by the third carrier (7.5 MHz), the output of CP-based CFR includes peaks that are above the threshold.
Some examples of prior art systems are: Lashkarian et al., “Crest Factor Reduction in Multi-Carrier WCDMA Transmitters,” 2005 IEEE 16th International Symposium on Personal, Indoor, and Mobile Communications, Vol. 1, pp. 321-325, Sep. 11-15, 2005; U.S. Patent Pre-Grant Publication No. 2004/0052314; and Sperlich et al., “Power Amplifier Linearization with Digital Predistortion, and Crest Factor Reduction,” IEEE MTT-S Int. Microwave Symposium Dig., Vol. 2, pp. 669-672, 2004.